1. Technical Field
Apparatuses and devices consistent with the present invention relate to discharge bulbs, and more particularly, to mercury-free discharge bulbs used as a light source for an vehicle headlamp.
2. Related Art
A related art discharge bulb, which includes an arc tube made of glass, has been generally used as a light source of a vehicle headlamp. However, the glass tube of the arc tube is corroded by the filled light-emitting material (e.g., metal halide). The blackening or devitrification caused by this corrosion prevents the discharge bulb from obtaining an adequate light distribution, and accordingly a life span of a vehicle headlamp in which the related art discharge bulb is used is decreased.
JP-A-2004-362978 describes a related art discharge bulb including an arc tube. A light-emitting portion of the arc tube is formed of a ceramic tube. The arc tube is filled with a light-emitting material and starting noble gas, and thin tube portions formed at both ends of the light-emitting portion and electrode bars inserted into the thin tube portions are bonded by frit glass so as to maintain an airtight condition within the arc tube. The frit glass bond thus seals both ends of the light-emitting portion of the arc tube. Further, when the electrode bar is bonded to the thin tube portion, a minute gap communicating with the discharge arc chamber is formed between the inside of the thin tube portion and the electrode bar. Since the ceramic tube does not react much with the filled light-emitting material (e.g., metal halide), the life span of the arc tube body made of ceramic is longer than that of the arc tube made of glass.
The discharge arc chamber of the related art discharge bulb is generally filled with an alkali metal halide (e.g., sodium halide, NaI) and a rare-earth metal halide (e.g., scandium halide, ScI3) as a main light-emitting material.
A discharge bulb for a vehicle headlamp naturally requires the quick rise of the luminous flux so that a predetermined luminous flux may be obtained directly after lighting. Accordingly, a starting performance of the discharge bulb is improved by filling the discharge bulb with a noble gas at high pressure as compared to a general discharge bulb. Further, the discharge bulb also requires that chromaticity is not significantly changed directly after lighting and when the discharge bulb is in a stable state. The related art discharge bulb described in JP-A-2004-362978 includes an arc tube that is formed of a cylindrical ceramic tube. Further, in order to form a compact bulb for an vehicle headlamp, the length of a region of the light-emitting portion is set in the range of 6 mm to 14 mm, the inner diameter of the light-emitting portion is set in the range of 1 mm to 3 mm, and the volume of an enclosed space is formed to be relatively small.
However, there are some disadvantages in the above-described related art discharge bulb. For example, even though the discharge arc chamber is filled with an appropriate amount of the light-emitting material, the luminous flux rises only slowly directly after lighting. Moreover, in the discharge bulb, the chromaticity changes widely from a time from directly after lighting until light emission becomes stable.